A fluoropolymer is produced by such methods as emulsion polymerization, suspension polymerization, solution polymerization and bulk polymerization, among others. Among them, an emulsion polymerization method is advantageous in a heat removability during polymerization and a low torque for stirring, since the volume efficiency of the polymerization vessel is high.
From an economical viewpoint, among others, it is preferred that the unreacted monomer or monomers not consumed in the polymerization reaction in emulsion polymerization be recovered for reuse thereof. Among such unreacted monomers, monomers which are gaseous at ordinary temperature, for example tetrafluoroethylene [TFE], can be readily separated and recovered as a gaseous phase after polymerization. On the other hand, it is not easy to separate and recover unreacted monomers which are liquid at ordinary temperature from the aqueous dispersion resulting from emulsion polymerization.
Known as a recovering method of unreacted monomers which are liquid at ordinary temperature is a method comprising recovering the unreacted monomer or monomers from the latex obtained by emulsion polymerization of a fluorinated monomer or monomers using a chlorofluorinated saturated hydrocarbon (CFC) as an extraction solvent (cf. e.g. Patent Document 1), the method comprising using, as an extraction solvent, such a fluorinated solvent as CF2ClCF2CFHCl, CF3CF2CHCl2, a hydrofluorocarbon or a fluorocarbon (cf. e.g. Patent Document 2) and the method comprising using a hydrofluoroether compound as an extraction solvent (cf. e.g. Patent Document 3).
When unreacted monomer separation is carried out using an extraction solvent, however, a problem arises; namely the extraction solvent remains in an aqueous dispersion after solvent extraction and causes troubles in an subsequent steps of processing. For avoiding such problem, it is necessary to evaporate the extraction solvent remaining in the aqueous dispersion (cf. e.g. Patent Document 4). For unreacted monomer reutilization, it is also necessary to isolate an unreacted monomer or monomers dissolved in the extraction solvent. For avoiding such economical and time burdens, it is preferred that unreacted monomer recovery can be made without using any extraction solvent.
A fluoropolymer obtained by emulsion polymerization of a fluoromonomer having a sulfonic acid precursor functional group convertible to a sulfonic acid group, for example —SO2F, is known to be useful as a electrolyte membrane for fuel cells, chemical sensors and so forth. Such a sulfonic acid precursor functional group-containing fluoropolymer obtained in the form of an aqueous dispersion by emulsion polymerization, in the condition of an aqueous dispersion, undergoes the reaction converting, by hydrolysis by water, a slight proportion of the sulfonic acid precursor functional group to a sulfonic acid group (—SO3M; M representing H, NR1R2R3R4 or M11 /Lwhere R1, R2, R3 and R4 are the same or different and each represents H or an alkyl group containing 1 to 4 carbon atoms and M1 represents a metal having a valence of L) (cf. e.g. Patent Document 5).
Therefore, a problem arises; namely, the hydrolysis gives the sulfonic acid group (—SO3M) lower in thermal stability than the sulfonic acid precursor functional group, causing thermal decomposition-due discoloration in the step of melt molding of the fluoropolymer. Another problem also arises; namely, even when the sulfonic acid-derived group-containing fluoropolymer is prepared in the form of a solid such as a powder, pellets or a molded article via drying treatment, the polymer immediately absorbs moisture in the air since the sulfonic acid groups (—SO3M) are highly hygroscopic, with the result that the moisture absorbed may cause foaming in the step of melt molding.
Sulfonic acid precursor functional group-containing fluoropolymers can be inhibited from undergoing discoloration and/or causing foaming in the step of melt molding by bringing them into contact with a fluorine radical-generating compound so that such unstable groups as —SO3M and —COOH at polymer chain termini may be fluorinated. However, since —SO3M groups exist abundantly, another problem arises; namely a water absorbed inhibits the fluorination.
There is a further problem; namely, the sulfonic acid precursor functional groups occurring in the fluoromonomer are hydrolyzed, so that a rate of unreacted monomer recovery is reduced.    [Patent Document 1] Japanese Kokai Publication S56-59810    [Patent Document 2] Japanese Kokai Publication H07-118332    [Patent Document 3] Japanese Kokai Publication H11-35624    [Patent Document 4] Japanese Kokai Publication 2000-344825    [Patent Document 5] International Publication WO 2005/028522